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I S I T T I M E T O M O V E U P T O A N H M C ?
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percent of all horizontals come with an APC
Tombstones are upright fixture blocks with
two or more sides onto which parts can be
mounted for machining. They are also known
as pedestal-type fixtures, tooling towers or
tooling columns. These fixture blocks can be
configured with two, three, four or six sides.
The basic idea of a tombstone is to hold
multiple parts per side. Typically, tombstones
are made from aluminum or cast iron, and
have bolt holes for attaching fixtures and
clamps to hold the parts. The tombstone
can be mounted on a moveable pallet to be
shuttled in and out of the machine.
Single-sided angle plates can also be mounted
on an HMC, with or without the use of a
tombstone, to machine parts made from
flat plate.
FIXTURING
With an HMC, you have to be more creative
when it comes to fixture designs. Fixtures may
require more sophistication than those for a
VMC, depending on part size. While it's often
easy to get by with using vises on a VMC,
vises are less useful on a horizontal. Common
tooling items used on HMC fixturing include
edge clamps, toe clamps and wedge clamps.
While fixturing on an HMC might seem
complicated, it also provides considerable
flexibility. HMCs can use hydraulic or
pneumatic fixturing. Some machines have
the option to plumb hydraulic or pneumatic
lines through the pallet which can then be
incorporated into the fixturing. M codes are
used in the CNC program to apply clamping
pressure into fixtures in the machining zone.
Although you can do the same on a VMC, it's
more convenient on an HMC, because the
pallet can be shuttled to the unload station
while the clamps are still engaged. Hydraulic
and pneumatic lines are usually fixed on a
VMC, but on a horizontal, the lines are usually
integrated in the pallet. This feature gives
you the ability to use multiple hydraulic or
pneumatic fixtures across multiple pallets
without having to remove and replace
hydraulic lines or fittings.
PALLET POOLS
Pallet pools enable more than two pallets to
be used for continuous operation. Without a
pallet pool, there are only two pallets and they
must run one after the other. A pallet pool
interfaces directly to the APC, enabling a pallet
to run in any order you decide. For example,
if you have four different jobs lined up on the
machine and one job runs out of material,
production of the other three can continue.
Typically, pallet pools consist of six or more
pallets, making around-the-clock, lights-out
operation possible. Even if you buy an HMC
with no intention of running all night, you
still gain savings by automatic loading and
unloading without interrupting the spindle
of the machine. The uptime created by this
method accounts for increased spindle
utilization, which can be as high as 85 percent.
CHIP EVACUATION
HMCs are designed to run long periods
without interruption. What happens to all
those chips?
With a high-pressure, through-the-spindle
coolant system, chip evacuation on an HMC
is easier than on a VMC. With a horizontal
spindle, chips naturally drop into the
conveyor.
Another benefit of gravity and high-pressure
coolant on an HMC is that deep-pocket milling
or drilling causes less wear on the tool and
preserves the surface finish. Chips have a
tendency to fall out of a pocket as they form
rather than remain in the bottom to interfere
with cutting action.
Select the best kind of chip conveyor based
on the type of work you plan to do with the
machine. For example, if you will be doing